1. Field of the Invention
The present invention relates to optical transmission apparatuses and, more specifically, to an optical transmission apparatus capable of efficiently transmitting an optical signal of large power with better transmission characteristics through an optical fiber by suppressing stimulated Brillouin scattering (SBS).
2. Description of the Background Art
As known, in optical fiber transmission, when an optical signal with large optical power of 10-odd dBm is supplied to an optical fiber, part of the optical signal is reflected due to stimulated Brillouin scattering (SBS) within the optical fiber, and as a result, optical power of an output optical signal is decreased.
A conventional optical transmission apparatus capable of transmitting an optical signal of large power with better transmission characteristics through an optical fiber is disclosed in xe2x80x9cOptical Communications Systemxe2x80x9d of Japanese Patent Laying-Open No. 9-69814 (1997-69814).
FIG. 9 is a block diagram showing the configuration of the conventional optical transmission apparatus disclosed in the above publication.
In FIG. 9, the conventional optical transmission apparatus includes a pilot signal generation part 100, an electro-optic conversion part 101, an optical fiber 102, and an opto-electric conversion part 103.
The pilot signal generation part 100 generates a pilot signal. The electro-optic conversion part 101 converts an input signal (electric signal to be transmitted) and the pilot signal into an optical signal through direct intensity modulation. The optical fiber 102 guides the optical signal. The opto-electric conversion part 103 converts the optical signal into an electric signal.
Described below is the operation of the conventional optical transmission apparatus.
First, the electro-optic conversion part 101 is supplied with an input signal and a pilot signal, and converts these signals into an optical signal. The optical signal outputted from the electro-optic conversion part 101 goes through the optical fiber 102 to the opto-electric conversion part 103 . The opto-electric conversion part 103 re-converts the received optical signal into an electric signal.
In the above operation, when electro-optic conversion is performed through direct intensity modulation, the optical signal obtained by conversion is not only intensity-modulated but also frequency-modulated (chirping). Multiplexing the input signal with a pilot signal having a frequency lower than that of the input signal causes the spectrum of the optical signal to spread over a wide band due to frequency modulation, thereby resulting in reduction in power of a main mode (maximum optical carrier component). SBS does not occur when the power of the main mode becomes less than a predetermined threshold.
The conventional optical transmission apparatus thus suppresses SBS by multiplexing the input signal with the pilot signal.
Furthermore, in the above publication, multiplexing the input signal with the pilot signal poses two problems: deterioration of transmission characteristics, and occurrence of intermodulation distortion between the input signal and the pilot signal. To solve the former problem, according to the publication, it is preferable to set the optical modulation index of the pilot signal to an appropriate value for suppressing SBS. To solve the latter, it is preferable to set the frequency of the pilot signal less than half of a Brillouin gain band.
However, in the former problem, the appropriate value for suppressing SBS is not necessarily clear and, therefore, a good transmission characteristic can not be ensured.
Furthermore, in the latter, the intermodulation distortion is not sufficiently eliminated only by setting the frequency of the pilot signal less than half of the Brillouin gain band.
Therefore, an object of the present invention is to provide an optical transmission apparatus capable of suppressing SBS by multiplexing an input signal with a pilot signal and further eliminating an adverse effect of intermodulation distortion between the input signal and the pilot signal, thereby allowing efficient transmission of an optical signal of large power with better transmission characteristics through an optical fiber.
The present invention has the following features to solve the problems above.
A first aspect of the present invention is directed to an optical transmission apparatus transmitting an optical signal through an optical fiber. The apparatus in accordance with the first aspect of the present invention comprises a pilot signal generation part for venerating a pilot signal. a multiplex part for multiplexing an input signal (electric signal to be transmitted) with the pilot signal, an electro-optic converter for converting an electric signal outputted from the multiplex part into an optical signal through direct intensity modulation, the optical fiber for guiding, the optical signal outputted from the electro-optic conversion part, and an opto-electric conversion part for converting the optical signal guided by the optical fiber into an electric signal. When the input signal is a frequency-multiplexed signal obtained by multiplexing a plurality of signals aligned on a frequency axis at regular intervals xcex94f (xcex94f greater than 0), the pilot signal generation part venerates the pilot signal having a frequency {mxe2x88x92(1/2)}xc3x97xcex94f (m is an arbitrary natural number).
In the first aspect, intermodulation distortion (second order distortion) between each signal included in the input signal and the pilot signal occurs exactly at the center between two consecutive signals on the frequency axis. Therefore, of the intermodulation distortion between the input signal and the pilot signal, second order distortion, which deteriorates transmission characteristics most, can easily be cut off by a filter. It is thus possible to suppress SBS by multiplexing the input signal with the pilot signal, and also to effectively eliminate an adverse effect of the intermodulation distortion between the input signal and the pilot signal. Consequently, an optical signal of large power can be efficiently transmitted with better transmission characteristics through the optical fiber.
According to a second aspect, further to the first aspect, a frequency of each signal included in the input signal is set to {nxe2x88x92(1/2)+C}xc3x97xcex94f (n=1, 2, 3, . . . and C is 0 or an arbitrary positive integer).
In the second aspect, the intermodulation distortion (second order distortion) between the signals included in the input signal occurs exactly at the center between two consecutive signals. Therefore, such second order distortion can also be cut off when the intermodulation distortion (second order distortion) between the input signal and the pilot signal, which occurs also exactly at the center between two consecutive signals, is cut off by the filter.
A third aspect of the present invention is directed to an optical transmission apparatus transmitting an optical signal through an optical fiber. The apparatus in accordance with the third aspect of the present invention comprises a pilot signal generation part for generating a pilot signal, a level adjusting part for adjusting a level of the pilot signal, a multiplex part for multiplexing an input signal (electric signal to be transmitted) with the pilot signal level-adjusted by the level adjusting part, an electro-optic conversion part for converting an electric signal outputted from the multiplex part into an optical signal through direct intensity modulation, the optical fiber for guiding the optical signal outputted from the electro-optic conversion part, an opto-electric conversion part for converting the optical signal guided by the optical fiber into an electric signal, a reflected light monitoring part for monitoring reflected light caused by stimulated Brillouin scattering (SBS) in the optical fiber, and an optical coupler for supplying the reflected light propagating through the optical fiber in a reverse direction to the optical signal to the reflected light monitoring part. Further, the level adjusting part adjusts the level of the pilot signal to a minimum value for suppressing the SBS based on monitoring results of the reflected light monitoring part. Still further, when the input signal is a frequency-multiplexed signal obtained by multiplexing a plurality of signals aligned on a frequency axis at regular intervals xcex94f(xcex94f greater than 0). the pilot signal generation part generates the pilot signal having a frequency {mxe2x88x92(1/2)}xc3x97xcex94f (m is an arbitrary natural number).
In the third aspect, intermodulation distortion (second order distortion) between each signal included in the input signal and the pilot signal occurs exactly at the center between two consecutive signals on the frequency axis. Therefore, of the intermodulation distortion between the input signal and the pilot signal, second order distortion, which deteriorates transmission characteristics most, can easily be cut off by a filter. It is thus possible to suppress SBS by multiplexing the input signal with the pilot signal, and also to effectively eliminate an adverse effect of the intermodulation distortion between the input signal and the pilot signal. Consequently, an optical signal of large power can be efficiently transmitted with better transmission characteristics through the optical fiber. Furthermore, adjusting the level of the pilot signal to an appropriate value, and more specifically, to a minimum value for suppressing SBS, brings far better transmission characteristics.
According to a fourth aspect, further to the third aspect, the reflected light monitoring part monitors the reflected light by measuring intensity thereof.
A fifth aspect of the present invention is directed to an optical transmission apparatus transmitting an optical signal through an optical fiber. The apparatus in accordance with the fifth aspect of the present invention comprises a pilot signal generation part for generating a pilot signal, an electro-optic conversion part for converting the pilot signal into an optical signal through direct intensity modulation, an external optical modulation part for intensity-modulating the optical signal outputted from the electro-optic conversion part with an input signal (electric signal to be transmitted), the optical fiber for guiding the optical signal outputted from the external optical modulation part, and an opto-electric conversion part for converting the optical signal guided by the optical fiber into an electric signal. When the input signal is a frequency-multiplexed signal obtained by multiplexing a plurality of signals aligned on a frequency axis at regular intervals xcex94f (xcex94f greater than 0), the pilot signal generation part generates the pilot signal having a frequency {mxe2x88x92(1/2)}xc3x97xcex94f (m is an arbitrary natural number).
In the fifth aspect, intermodulation distortion (second order distortion) between each signal included in the input signal and the pilot signal occurs exactly at the center between two consecutive signals on the frequency axis. Therefore, of the intermodulation distortion between the input signal and the pilot signal, second order distortion, which deteriorates transmission characteristics most, can easily be cut off by a filter. It is thus possible to suppress SBS by multiplexing the input signal with the pilot signal, and also to effectively eliminate an adverse effect of the intermodulation distortion between the input signal and the pilot signal. Consequently, the optical signal of large power can be efficiently transmitted with better transmission characteristics through the optical fiber.
A sixth aspect of the present invention is directed to an optical transmission apparatus transmitting an optical signal. The apparatus in accordance with the sixth aspect of the present invention comprises a pilot signal generation part for generating a pilot signals a multiplex part for multiplexing an input signal (electric signal to be transmitted) with the pilot signal, and an electro-optic conversion part for converting an electric signal outputted from the multiplex part into an optical signal through direct intensity modulation. When the input signal is a frequency-multiplexed signal obtained by multiplexing a plurality of signals aligned on a frequency axis at regular intervals xcex94f(xcex94f greater than 0), the pilot signal generation part generates the pilot signal having a frequency {mxe2x88x92(1/2)}xc3x97xcex94f (m is an arbitrary natural number).
According to a seventh aspect, further to the sixth aspect, a frequency of each signal included in the input signal is set to {nxe2x88x92(1/2)+C}xc3x97xcex94f (n=1, 2, 3, and C is 0 or an arbitrary positive integer).
An eighth aspect of the present invention is directed to an optical transmission apparatus transmitting an optical signal through an optical fiber. The apparatus in accordance with the eighth aspect of the present invention comprises a pilot signal generating part for generating a pilot signal, a level adjusting part for adjusting, a level of the pilot signal, a multiplex part for multiplexing an input signal (electric signal to be transmitted) with the pilot signal level-adjusted by the level adjusting part, an electro-optic conversion part for converting an electric signal outputted from the multiplex part into an optical signal through direct intensity modulation. a reflected light monitoring part for monitoring reflected light caused by stimulated Brillouin scattering (SBS) in the optical fiber. and an optical coupler for supplying the reflected light propagating through the optical fiber in a reverse direction to the optical signal to the reflected light monitoring part. The level adjusting part adjusts the level of the pilot signal to a minimum value for suppressing the SBS based on monitoring results of the reflected light monitoring part. Further. when the input signal is a frequency-multiplexed signal obtained by multiplexing a plurality of signals aligned on a frequency axis at regular intervals xcex94f (xcex94f greater than 0). the pilot signal generation part generates the pilot signal having a frequency {mxe2x88x92(1/2)}xc3x97xcex94f (m is an arbitrary natural number).
According to a ninth aspect, further to the eighth aspect, the optical transmission apparatus further comprises an optical coupler for supplying the reflected light propagating through the optical fiber in a reverse direction to the optical signal to the reflected light monitoring part.
A tenth aspect of the present invention is directed to an optical transmission apparatus transmitting an optical signal. The apparatus in accordance with the tenth aspect of the present invention comprises a pilot signal generation part for generating a pilot signal, an electro-optic conversion part for converting the pilot signal into an optical signal through direct intensity modulation, and an external optical modulation part for intensity-modulating the optical signal outputted from the electro-optic conversion part with an input signal (electric signal to be transmitted). When the input signal is a frequency-multiplexed signal obtained by multiplexing a plurality of signals aligned on a frequency axis at regular intervals xcex94f (xcex94f greater than 0) the pilot signal generation part generates the pilot signal having a frequency {mxe2x88x92(1/2)}xc3x97xcex94f (m is an arbitrary natural number).